Problem: Omar ate 3 slices of cake. Stephanie ate 1 slice. If Omar ate $\dfrac{3}{7}$ of the cake, what fraction of the cake is remaining?
If 3 slices represent $\dfrac{3}{7}$ of the cake, there must have been a total of 7 slices. $\text{fraction of cake remaining} = \dfrac{\text{number of slices remaining}}{\text{number of slices total}}$ Together they ate 4 slices, which leaves 3 out of 7 slices remaining. There is $\dfrac{3}{7}$ of the cake remaining.